Removing Plaster from Mold

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debbie t
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Joined: Tue Mar 23, 2010 5:11 pm

Removing Plaster from Mold

Post by debbie t »

Hi I have been casting for 5 years now. Today I am creating very large open face castings. Big pieces pose new problems. After removing most all of the plaster I am still left with surface plaster in some spots. Since the piece is very large it really does not fit into my available sandblaster. I have tried a lot of things over the years and I am looking for new tips on how to remove excess surface plaster. It looks like a thin light dusting on the surface in spots and crevices. It is difficult to remove even with hand tools and pads. Another question would be: is there a way to prevent the plaster from sticking in the first place, and make it easier to remove? I use R&R casting plaster.
Thanks in advance for your help , Debbie T
Jenna
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Re: Removing Plaster from Mold

Post by Jenna »

See post below...
Last edited by Jenna on Thu Jan 15, 2015 8:27 pm, edited 1 time in total.
Jenna
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Re: Removing Plaster from Mold

Post by Jenna »

I assume you're using some kind of kiln wash as a separator? After doing many tests with all the different brands, Primo is my absolute favorite. Mr97 also works great.
As for removing the sticking plaster, have you tried soaking your pieces in CLR? That stuff is amazing and you can use it over and over. I've been using the same bottle for about two years. It gets into all the crevices and after an overnight soak, a scrub with a nice stiff brush is all I need to get my castings really clean.
How big are your pieces? How deeply embedded is the plaster?
debbie t
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Re: Removing Plaster from Mold

Post by debbie t »

Hi Jenna,
Thank you for your quick response. I was so happy to see your e-mail. I haven’t tried a kiln wash on my molds. I just place my Bullseye frit right into the clean plaster mold. I will try the kiln wash and CLR. How much kiln wash do you apply? The pieces I am making are very big over 30 inches in diameter. I can probably soak the CLR using a saturated towel. This sounds very good and I will post my results. A lot of the plaster is imbedded lightly into the surface but enough to make it stubborn also there are many undercuts on my piece and recessed areas so a real challenge in both casting and refining. Most of my pieces are refined by hand over many weeks and I don’t like to rely totally on sandblasting because it doesn’t allow for that hand finished look that enlivens the piece. Thank you again for your kindness,
Best,
Debbie Tarsitano
Jenna
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Re: Removing Plaster from Mold

Post by Jenna »

Hi Debbie,
My molds are much smaller than yours but here is what works for me...
Mix the primer well and pour it into the mold. Swirl it around for a second or two and then dump the primer back into your container. You can re-use this primer again and again. I also use a small artist paintbrush ( wetted with primer) to wick or guide any excessive primer out of details, although this is not entirely necessary. This method gives me enough coverage without any brush marks in the finish. You only need to do this once. More is not better. You only need a very thin layer. Cynthia has written that she also uses a similar method but has mentioned that It works better for her when the mold is still slightly damp. I let the primer dry before loading the mold with glass. This only takes a few hours of air drying or you can use a hair dryer to speed things up. No need to prefire the primer in the kiln, from my experience.

If you do decide to use Primo, here are a few things that have made it work better for me.

Mix your primer a day or two before you need it, if possible. This lets all the dry powder absorb into the water. A milk frother can really help with the initial mixing. You'll notice the powder just floating on the surface.

Use distilled water.

The one thing about primo is that it settles into the bottom of the container into a thick heavy muck which can be really hard to stir up. I pour off the liquid into a separate container, use a spoon to break up the muck in the bottom, then pour the liquid back in and mix. It's also useful to put some marbles into the container to help break up the muck as it settles while you're using it.

Primo uses some purple colorant to dye the primer. I find that these dye crystals don't always dissolve fully and strain them out using a reusable coffee filter. The filter will let all the other stuff thru and trap just the crystals.

Most of this advice would be the same for any other primer. Many glass artists swear by Bullseye primer.
Hope this helps and I hope it cuts down on cold working. I strive to do little to no coldworking. If you need any clarification, just ask.
Jenna
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Re: Removing Plaster from Mold

Post by Jenna »

I just re-read your reply. 30 inches is a really big piece. I'm thinking MR-97 might be easier to use than trying to dump the kiln wash back out of your mold. I'm sure you prefer to handle your molds as little as possible. It is an aerosol so the can needs to be sprayed while upright which is one reason it is impractical for me and my many small round molds. But for one large mold, it might be the way to go. Expensive, but it works really well.
debbie t
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Joined: Tue Mar 23, 2010 5:11 pm

Re: Removing Plaster from Mold

Post by debbie t »

Hi Jenna,
Thanks for the help. I purchased the CRL today and will experiment with it. I will look into the primers and try them out too. Thank you for explaining how to use them. I wouldn’t have known where to start. When I have some results I will post them. It is a difficult process and it is true if you can do as little cold working as possible that is the best, but there is always the need for some. It is a long process to create a cast glass sculpture and probably the most difficult process in all of glass art.
Thank you again and have a nice weekend,
Best, Debbie Tarsitano
Morganica
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Re: Removing Plaster from Mold

Post by Morganica »

Well...I spent years and years trying to figure out how to get spent mold material to release cleanly from a mold, discovered a lot of tricks (one is eliminate as much silica as possible from your mold mix, and a dollop of kilnwash in the face coat is another).

But I also learned that there's definitely a point of diminishing returns. Sufficient MR-97 to cover multiple 30-inch molds is a fair chunk o'change and won't completely solve the problem anyway (and I had one case where I wound up having to figure out how to remove BN residue from the casting, which was irritating).

The smarter approach is to become more efficient at post-firing mold removal. I'm not sure CRL is the answer, either. I used to use it, but occasionally had surface issues if I left the piece in too long, and found it a pain to handle as much as was needed for big castings. Mechanical methods generally work better for me.

Ultimately, I prefer using a power washer. A nice one costs about $100 or so, and you will be astonished at how well it cleans your castings. What used to take 4-6 hours to completely clean now takes about 10 minutes (not kidding) and I have never had it break the glass. (Caution: These things are POWERFUL, so read the manual and follow it carefully. I accidentally brushed mine over my gloved finger and nearly lost a knuckle.)

You'll still need to coldwork scum/devit; all a powerwasher will do is get it extremely clean.

I do occasionally wind up with refractory down in a bubble pocket or tucked into a very tight undercut where the washer can't reach, and sometimes my jewelry castings are too small for the washer. For those, I have two different devices, both of which I picked up on eBay:

--Ultrasonic cleaner. This is a larger version of a jewelry cleaner, and it's wonderful for smaller castings. You fill it with water and a cleaning solution (I use Dawn detergent), turn it on and let it run 2-4 hours. It bombards the casting with sound waves and knocks off the gunk.

--Ultrasonic dental pick. You can pick them up relatively inexpensively on eBay, and they're exactly what the dental hygienist uses to clean your teeth. You touch the needle-like probe to the crevice, it shoots out sound waves with a thin stream of water and literally pulverizes ONLY the plaster, then washes it out of the crevice.

Bottom line, though: I agree with Debbie and Richard Whiteley, who said something on the order of glass you pull out of the kiln is only half done. If you cast, you coldwork. There's really no way to avoid it.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
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"I wrote, therefore I was." (me)
charlie
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Re: Removing Plaster from Mold

Post by charlie »

take it to the car wash and use their washer. it's not as powerful as a purchased power washer, because no one likes removing the paint from their car.
Bert Weiss
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Re: Removing Plaster from Mold

Post by Bert Weiss »

charlie wrote:take it to the car wash and use their washer. it's not as powerful as a purchased power washer, because no one likes removing the paint from their car.
At $1, this is the cheapest tool, I know of, to rent time on. Get a nice piece of thick foam and take it and the glass to the car wash!
Bert

Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
debbie t
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Joined: Tue Mar 23, 2010 5:11 pm

Re: Removing Plaster from Mold

Post by debbie t »

Hi ,
The dentil pic sounds like a good idea for the tight spaces. I tried an electric tooth brush, a friends idea, and believe it or not I was thinking of some of the abrasive tooth pastes they are making these days which have a grit in them but are gentle. I am going to try that next. The CRL seemed to work I put it on carefully with a tooth brush, scrubbing it in and letting it sit for 2 minutes then cleaned it several times with water and water soaked q-tips and paper towels to neutralize the CRL.(cheap q tips are at the dollar store). I use a lot of q-tips and tooth pics on my molds. I live in a very cold climate and have limited water inside my studio, so some ideas with lots of water will not work for me. In the summer outside that would be great on the very large pieces. A power washer sounds good but sadly it will have to wait for spring.
I really appreciate all the advise and sharing.
A little of everything plus lots of patience will eventually work. It will probably take me the next two months to refine this casting. I made other sculptures and cold worked them by hand for months that came out looking like marble. As you work the piece you get to know it and I really believe the sculpture comes to life when you cold work it by hand over time even though it is very hard to do. The work reveals itself and becomes mature going from a lump of glass to a radiant sculpture. For many years (over 30) I was a flameworker and hot glass sculptor which many people know is very time consuming work. This is much more difficult and complex but the rewards are great!

Thank you for the great conversations. Debbie Tarsitano
Morganica
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Re: Removing Plaster from Mold

Post by Morganica »

Hi, Debbie;

Yep, familiar with (and like) your flameworked paperweights, saw many at Corning (do you live near there?).

You're right, a power washer is pretty much a 3-season tool if you live in areas with sustained freezes. When it's frozen here (not that often, fortunately), I tend to use the sonic pic a lot more. I don't have a water source in my garage/studio, so I use the sonic pic with a bucket and water pump, works just fine. The sonic cleaning baths can be filled from inside the house (I tend to fill gallon jugs with clean water and bring them into the garage to stand by until needed when it's freezing; otherwise, I bring a hose in from outside and hook it up to loc-lines).

I started out casting larger sculptures with the notion that I'd eventually turn out perfect pieces straight from the mold, exactly as they were in the wax. I actually got there a couple of times...and was surprised to discover that I didn't much like the result. Discovered that I needed to see the piece out of the mold to really be able to carve in the detail I wanted. And, as much as I thought I hated coldwork, I preferred the control I got over the final appearance when I hand=shaped with the grinding tools.

Odd, that.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
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"I wrote, therefore I was." (me)
debbie t
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Joined: Tue Mar 23, 2010 5:11 pm

Re: Removing Plaster from Mold

Post by debbie t »

I am going to use the sonic pic will get one this week. I know what you mean, I have been schlepping water bottles for years. It is a simple solution to the problem of no water in the studio. I always feel like a pioneer on the prairie. I wish I lived closer to Corning 7 hours away. They have everything needed there and are always so helpful. I made the large casting last month using Corning’s kiln. I am an instructor there. So it is made in Corning and I like that idea. I don’t mind the hand finish I think it will come out better too yet I will still be on the look out for better tools and ideas. I think it takes a large variety of tools to cold work castings because of the shapes, sizes,thinness, thickness and weight. One piece can have such variety of surfaces too.
Thanks again,Take Care,
Deb
Bert Weiss
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Re: Removing Plaster from Mold

Post by Bert Weiss »

I picked up a backpack sprayer, one day, on sale cheap. This is a pretty good water delivery device for using my diamond skil saw or wet polishing wheels.
Bert

Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
debbie t
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Joined: Tue Mar 23, 2010 5:11 pm

Re: Removing Plaster from Mold

Post by debbie t »

I was wondering which sonic pic did you purchase and how much did you pay for it. There are a few out there.
Thanks,
Debbie Tarsitano
Morganica
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Re: Removing Plaster from Mold

Post by Morganica »

I got it on eBay a few years back, a DTE piezo ultrasonic dental scaler is I think the proper name. You can get them for somewhere between $110-$175. If you want the fancier models with built-in water reservoirs and more settings/features, they're about 3-4 times that much.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)
debbie t
Posts: 9
Joined: Tue Mar 23, 2010 5:11 pm

Re: Removing Plaster from Mold

Post by debbie t »

Hi This is a lot to ask but can you go to e-bay and let me know which one looks like yours. I Saw them there but wouldn’t know which one it is. Thank you,
Best, Deb
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